Pesticides are employed in agriculture to improve crop yields by controlling pests such as weeds, insects, nematodes and fungi which attack plants and transmit diseases. Pests result in an estimated total annual loss to U.S. agriculture of about 10% of production, and an estimated worldwide agricultural losses of about 14% of production. Pesticides, however, require repeated application at targeted sites during the growing season to replace losses due to volatilization, leaching and degradation. Repeated pesticide application, however, is undesirable because of high costs and possible phytotoxicity, and because certain pesticides are well known environmental pollutants. For a review on pesticides, see, Kirk-Othmer's "Concise Encyclopedia of Chemical Technology," John Wiley & Sons, publishers; New York, USA, 1985, pages 652-656.
Various pesticide compositions and methods for preparing the same have been developed in attempt to enhance pesticidal efficacy so that less pesticide may be used and to reduce the need for repeated application. Exemplary of such compositions include controlled-release pesticide formulations which has become increasingly popular due to safe handling and ease of application.
Conventional controlled-release formulations typically employ natural or synthetic polymers to uniformly disperse and encapsulate the pesticides. These formulations, however, suffer from a number of deficiencies which include ineffective control of the released amount of pesticide per unit time and control of the duration of the release period. Moreover, the compositions are suitable for encapsulating a limited number of pesticides. Accordingly, there is a need in the art for improved controlled released pesticide formulations which overcome one or more of the aforementioned deficiencies.
B. S. Shasha et al. ((1981) J. Polymer Science, Polymer Chemistry Edition, Vol. 19, pages 1891-1899) disclose encapsulation of oil soluble pesticides in a starch-calcium matrix by mixing the pesticides with alkali starch followed by precipitation of the mixture with a calcium chloride solution. D. Trimnell et al. ((1982) J. Applied Polymer Science, Vol. 27, pages 3219-3928) describe oil soluble pesticide encapsulation using a starch-borate complex by mixing starch, pesticide, and water to form a mixture; adding alkali to the mixture gelatinize the starch; and boric acid treatment of the gelatinized mixture. These methods, however, are not suitable for encapsulating pesticides which are alkali-sensitive. Moreover, the methods do not provide controlled release formulations having selective release rates of pesticides at the same pesticide loading.
B. S. Shasha et al. ((1984) J. Applied Polymer Science, Vol. 29, pages 67-73) studied herbicide S-ethyl dipropylthiocarbamate (EPTC) encapsulation in a starch-borate complex. The method involves mixing the herbicide into a neutral paste of pregelatinized starch or flour, followed by the addition of ammonia or an amine. The paste is subsequently treated with boric acid or borate salt to form a gel, which is then coated with additional starch to facilitate breaking of gel into particles for drying. While satisfactory yields of encapsulated products are obtainable from these methods, Shasha does not suggest any method for selectively controlling the pesticide release rate of the encapsulated products at the same pesticide loading.
F. H. Otey et al. ((1984) J. Agriculture and Food Chemistry, Vol. 32, pages 1095-1098) describes the use of a starch or starch-borate matrices for controlled release of urea fertilizer. B. S. Shasha et al. ((1989) J. Controlled Release, Vol. 19, Page 255) described urea pellets coated with starch containing entrapped herbicides. These papers, however, do not disclose nor suggest a method for encapsulating pesticides or a controlled-release pesticide composition having selective release rate properties.
Accordingly, there is a need in the art for improved controlled-release formulations for encapsulating a broad range of oil-soluble pesticides and which have controllable release rates and release periods, relative to conventional formulations, and a practical method for preparing the same.